This invention relates to a method capable of producing flake particles directly from a supply of molten material by the use of a rotating member having discrete serrations in the peripheral edge thereof in which the leading surfaces of the serrations contact the molten material and have formed thereon flake particles.
As used herein the terms flake particles, flakes and flake refer to particles of relatively small size, in the hundredths of an inch range, and the terms include particles frequently referred to as powders because of their small size.
A large number of methods exist for the manufacture of metal flakes or powders. These range from various mechanical methods, such as grinding or filing to casting methods utilizing water or jets to break up a molten metal stream.
In recent years, attention has been directed toward those methods which produce a very fine particle size or very fine grain size within the particle. It has been observed that, when grain sizes or particle sizes decrease below several microns, there are significant advantages to be gained in terms of ease of processing and the quality and properties of the product. Flake particles having grain size in this small range are equally useful for consolidation by hot isostatic compaction, sintering, hot extrusion, or hot forging and rolling processes which yield products with properties equivalent to or better than those of the wrought alloys.
The surface area of the flakes and powders are important also. While these fine powders are attractive, the huge surface areas created are readily contaminated, and handling becomes difficult. On the otherhand, larger particles with a very fine grain size are less easily contaminated, and retain many of the desirable characteristics of the very fine powders. It is desirable to produce a product which has a very fine grain size that is not so small as to have a large total surface area in a substantial quantity of individual product members. A discussion of this will be found in a paper published in Solidification Technology, pp. 317-336, NCIC in 1974.
Rapid quenching is probably the simplest method for producing small grain sizes. In general, higher quenching rates produce smaller grain sizes, with quench rates of the order of 10.sup.6 C. degrees change per second of time producing grain sizes (or dendrite arm spacing) of the order of one micron. For the most part, the standard gas or water atomization processes for powder manufacture are limited to quench rates below about 10.sup.4 C. per second, and therefore to dendrite arm spacing of 10 microns.
Splat quenching in which molten metal contacts a cool metallic surface provides quench rates that are very high. Splat quenching (cooling) has been practiced in the past by atomizing droplets against a rotating smooth cooling disc to produce powders of relatively uncontrolled configurations and irregular shapes, with a random distribution of variation in these parameters.
All of the prior art methods that are used to make flake particles have various deficiencies. For instance, a normal method would be to grind or chip or cut the ends of a wire or rod in progressive sectional slices, each ending up as a flake or powder particle. In this process, the wire or rod must be formed and mechanically worked to its appropriate cross section before the slicing operation, which is time-consuming and an extra expense. The slicing, grinding or chipping requires multiple tool faces which wear and become dull, requiring replacement, sharpening and other expensive treatments.
Other prior art methods using atomization and spraying are relatively uncontrolled from a particle size distribution and configuration standpoint. In these methods, an orifice is required which is a source of problems from a clogging and wear, etc., standpoint. In addition, the use of orifices have several attendant difficulties in that they must function in the severe environment of flowing molten metal. Where the molten metal product desired is composed of low-melting-point alloys, such as lead, tin, zinc, etc., the problems with the orifice are not severe. However, due to the commercial demand to continuously make a product out of materials having higher melting points, other processes using orifices are plagued with difficult problems.
The use of an orifice usually requires additional heating to insure that metal does not solidify in the orifice and thereby changes the shape of the product formed. The use of small orifices requires extremely clean melts to prevent intermittent plugging or restriction of the orifices.
The present invention forms the desired product directly from the molten state and without the need for controlling the size of the orifice or flow rate. Thus, though the advantages of high-quench cooling rates to produce flakes and powders are known, practical and controlled methods as well as apparatus for carrying this out are provided in this invention.
Although it is known in the laboratory to produce splat quench rates as high as 10.sup.7 -10.sup.9 C. degrees per second to produce grain sizes of less than 0.01 micron, from a practical commercial application, methods capable of producing the formed product are still being sought and have not been reported at quench rates above 10.sup.4 C. degrees per second.
In the present process the formation of the materials into final flake particles form is carried out while the material is formed directly from the molten state, and therefore inorganic compounds having properties in the molten state similar to that of molten metals and metal alloys may be formed in substantially the same manner. The properties that must be similar to those of molten metal are the viscosity and surface tension in the molten state, as well as the compound having a substantially discrete melting point, rather than the broad continuous range of viscosities characteristic of molten glasses.
Materials conforming to the class for this invention and having such properties will have a viscosity in the molten state when at a temperature of within 25% of their equilibrium melting point in degrees Kelvin in the range of 10.sup.-3 to 1 poise as well as having surface tension values in that same temperature range in the order of from 10 to 2500 dynes per centimeter.
The prior art discloses atomization of molten stream materials sprayed from an orifice upon the surface of a rotating copper roll. When the atomized stream strikes and splotches against the cool surface of the roll, rapid quenching takes place and a multitude series of random-shaped flakes are formed.
The present invention controls the shape and size of the final flake product. Controlling the size and shape, including thickness, are very important in determining the physical properties of the product when the size of the product is very small.